Tracking Deep Sediment Underplating in a Fossil Subduction Margin: Implications for Interface Rheology and Mass and Volatile Recycling

The architecture and mechanical properties of the subduction interface impact large‐scale subduction processes, including mass and volatile recycling, upper‐plate orogenesis, and seismic behavior. The nature of the deep subduction interface, where a dominantly frictional megathrust likely transitions to a distributed ductile shear zone, is poorly understood, due to a lack of constraints on rock types, strain distribution, and interface thickness in this depth range. We characterized these factors in the Condrey Mountain Schist, a Late Jurassic to Early Cretaceous subduction complex in northern California that consists of an upper and lower unit. The Lower Condrey unit is predominantly pelagic and hemipelagic metasediment with m‐to km‐scale metamafic and metaserpentinitic ultramafic lenses all deformed at epidote blueschist facies (0.7–1.1 GPa, 450°C). Major and trace element geochemistry suggest tectonic erosion of the overriding plate sourced all ultramafic and some mafic lenses. We identified two major ductile thrust zones responsible for Lower Condrey unit assembly, with earlier strain distributed across the structural thickness between the ductile thrusts. The Lower Condrey unit records distributed deformation across a sediment‐dominated, 2+ km thick shear zone, possibly consistent with low velocity zones observed in modern subduction zones, despite subducting along a sediment poor, tectonically erosive margin. Periodic strain localization occurred when rheological heterogeneities (i.e., km‐scale ultramafic lenses) entered the interface, facilitating underplating that preserved 10%–60% of the incoming sediment. Modern mass and volatile budgets do not account for erosive margin underplating, so improved quantification is crucial for predicting mass and volatile net flux to Earth′s interior.